This invention relates to a system for establishing the shape of mirrors for solar collectors, and more particularly to such a system that uses discrete actuators.
Very large mirrors and optical arrays are used in a number of applications. These applications include in-orbit large space telescopes for astronomical imaging and scientific earth observation systems. Large mirrors and optical arrays may be used in terrestrial systems as well, such as large and small solar energy concentrators. These systems often consist of precision manufactured segmented mirrors and panels as seen in FIG. 1. Constructing these systems from a single mirror is not practical because of their large size.
One example of an in-orbit telescope is the James Webb Telescope now being designed and built by NASA [1]. Numerals in brackets refer to the references included herewith. The contents of all of the references cited herein are incorporated herein by reference in their entirety. Segmented mirror telescopes such as the James Webb Telescope often have errors in their composite shape due to construction tolerances in the mirror supporting structure and disturbances such as thermal distortions, gravity, etc. Mirrors in large telescopes like the James Webb apparatus are equipped with complex continuous actuators to correct their shape.
An important terrestrial example is solar concentration systems such as those seen in FIG. 1. Manufacturing precision mirrors for solar concentrators and making them robust to disturbances such as wind and thermal warping is expensive. Imprecision in mirror shape due to manufacturing limitations and disturbances degrades the performance of solar concentrator systems [2-4]. Embedding simple actuators in the mirror structure to correct the shape can decrease manufacturing costs and improve system performance.
Using embedded actuators to actively control a structure's shape is complex and has been a topic of research. Researchers have designed systems and protocols to align segmented mirror arrays. In one example developed by MacDonald et al., the segmented array of small square mirrors is aligned using continuous, linear piezoelectric actuators [5]. In a second example, Ulich et al. developed a shape correction system for a hexagonal segmented mirrors array [6]. In this implementation, each mirror segment is adjusted by three continuous piston actuators.
Researchers have also designed deformable mirror systems with embedded continuous actuators to correct the mirror shape. One approach that has been applied to control mirror shape is to laminate continuous actuators to the rear of the deformable mirror [7,8]. Types of actuators that have been considered include MEMS and piezoelectric devices. Another approach that has been considered is mounting continuous actuators in a flexible mirror-supporting structure. Novak considered using continuous linear actuators directly connected to a flexible mirror under force control [9]. Gullapalli et al. developed a technique which used continuous actuators embedded in a hinged truss substructure to adjust the shape of a deformable mirror [10]. Other researchers such as Griffith et al. used a combination of continuous actuators embedded in the mirror and in the mirror substructure [11].
These prior art solutions have several drawbacks. First, systems containing continuous actuators are complex and require a large number of actuators, sensors and motion control systems. For example, an n segment system would require 7n active motion control subsystems to control the position, orientation and shape of each element. Further, these continuous actuators are heavy, a major issue for space systems. In addition, these systems have a high probability of failure. The hostile environments in which solar mirrors are used (space, deserts and seaside regions) degrade precision motion control elements and repairs are often difficult or impossible due to restricted access.
It is an object of the present invention to provide a simpler system for controlling the shape and position of solar mirrors. Yet a further object is a system for controlling the shape and position of solar mirrors having a precision approaching that of conventional continuously actuated systems.